Abstract

Suspensions of nanoparticles (i.e., particles with diameters < 100 nm) in liquids,
termed nanofluids, show remarkable thermal and optical property changes from the base
liquid at low particle loadings. Recent studies also indicate that selected nanofluids
may improve the efficiency of direct absorption solar thermal collectors. To determine
the effectiveness of nanofluids in solar applications, their ability to convert light
energy to thermal energy must be known. That is, their absorption of the solar spectrum
must be established. Accordingly, this study compares model predictions to spectroscopic
measurements of extinction coefficients over wavelengths that are important for solar
energy (0.25 to 2.5 μm). A simple addition of the base fluid and nanoparticle extinction
coefficients is applied as an approximation of the effective nanofluid extinction
coefficient. Comparisons with measured extinction coefficients reveal that the approximation
works well with water-based nanofluids containing graphite nanoparticles but less
well with metallic nanoparticles and/or oil-based fluids. For the materials used in
this study, over 95% of incoming sunlight can be absorbed (in a nanofluid thickness
≥10 cm) with extremely low nanoparticle volume fractions - less than 1 × 10-5, or 10 parts per million. Thus, nanofluids could be used to absorb sunlight with
a negligible amount of viscosity and/or density (read: pumping power) increase.